Dimensionality and Interface Engineering of 2D Homologous

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Dimensionality and Interface Engineering of 2D Homologous Perovskites for Boosted Charge Carrier Transport and Photodetection Performances Dejian Yu, Fei Cao, Yalong Shen, Xuhai Liu, Ying Zhu, and Haibo Zeng J. Phys. Chem. Lett., Just Accepted Manuscript • Publication Date (Web): 23 May 2017 Downloaded from http://pubs.acs.org on May 23, 2017

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The Journal of Physical Chemistry Letters

Dimensionality and Interface Engineering of 2D Homologous Perovskites for Boosted Charge Carrier Transport and Photodetection Performances Dejian Yu, Fei Cao, Yalong Shen, Xuhai Liu, Ying Zhu, Haibo Zeng*

Institute of Optoelectronics & Nanimaterials, MIIT Key Laboratory of Advanced Display Material and Devices, College of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing 210094, China

AUTHOR INFORMATION Corresponding Author E-mail: [email protected] (H. Zeng)

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ABSTRACT: Two dimensional (2D) homologous halide perovskites (HPs) microcrystallines have emerged as promising alternative light-sensitive materials, however, the undesirable quantum confinement effect and severe interfacial charge carrier scattering still hamper their applications in photodetectors (PDs). Here we propose a novel post-synthetic treatment to simultaneously solve both problems. 2D (OA)2FAn-1PbnBr3n+1 (OA and FA represent octadecylamine and formamidine) microplatelets film was immersed into solution containing FA+, leading to improvements in two aspects. Firstly, the dimensionality of 2D HPs was increased through exchange reaction between OA+ and FA+, which meliorates the quantum confinement effect and facilitates the separation of electrons and holes; secondly, the free-standing 2D HPs microcrystallines were fused for promoted inter-domain charge carrier transport. The treated PDs achieved 3600% and 4200% increase of external quantum yield and responsivity up to 7100% and 32 A/W, respectively. And the rise/decay time was shortened by two orders of magnitude down to 0.25/1.45 ms.

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Halide perovskites (HPs) have emerged as a superstar in optoelectronic and photovoltaic fields in recent years.1-8 They possess superior merits including high light absorption coefficient, large charge carrier diffusion length, long charge carrier lifetime and excellent defects tolerance.9-13 Solar cells based on hybrid HPs have advanced power conversion efficiencies up to 22.1%.5 Photodetector (PD) is another kind of important optoelectronic device, which has been widely used in fields of environmental

monitoring,

biological

sensing,

imaging

and

optical

communication.14-17 Intensive investigations have been devoted to integrating HPs into versatile PDs prototypes.18-21 Early researches mainly focused on polycrystalline film-sensitized PDs.22 The main problem of the film system is the large defects density at the grain boundaries,23-25 which play as trap centers and significantly recede the PDs performance. Soon afterwards, bulk single crystals with extremely low defects density were utilized as photoresponse materials.26,27 However, the uncontrollability of their growth into bulk made them incompatible with PDs fabrication techniques.

Besides,

these

PDs

usually exhibited narrow-band

photoresponse.26,27 In pursuit of high-performance PDs based on polycrystalline film, large domain size, preferred orientations and low intrinsic defect density were found to play major roles,28-31 which bring suppressed trapping and enhanced charge carrier transport. Interesting, while it is very difficult to improve the three parameters in polycrystalline system, films assembled with anisotropic HPs microcrystallines, especially two dimensional (2D) homologous HPs, which have been extensively explored in 3 ACS Paragon Plus Environment


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optoelectronic and photovoltaic applications,32-36 seemingly better satisfy such requirements. For 2D homologous HPs, basically a chemical formula of R2An-1PbnX3n+1 is adopted,37 where n is the number of octahedra layers and R represents long-chain ammonium that can not fit the cavity within the octahedra framework. Due to the 2D structural characteristic, large edge size up to several micrometers can be easily obtained,38-40 which is much larger than the domain size in a polycrystalline film and therefore increases charge carrier mean-free path and reduces hopping. Besides, the 2D HPs naturally possess uniaxial lattice orientation,41,42 which recently was reported to significantly promotes charge carrier transport and enhance photovoltaic performances.29,43 Moreover, 2D HPs own suppressed defect concentration and enhanced stability compared with other counterparts.44,45 Therefore, the 2D HPs microcrystallines hold great promise for PDs applications. For example, Tan et al fabricated PDs on individual 2D (C4H9NH3)2PbBr4 crystal with ultrahigh responsivity up to 2100 A/W.38 However, when these 2D HPs microcrystallines are assembled into film, two problems still significantly hamper the performances of PDs. Firstly, electrons and holes usually suffer from strong quantum confinement,41,46 and are therefore difficult to separate and be collected. Secondly, the inter-domain charge carrier transport is severely hindered by interfacial scattering effect. To further improve the performances of PDs, innovative solution is in urgent need. In this work, we report a novel post-synthetic treatment to simultaneously engineer the dimensionality and interface of 2D homologous HPs microcrystallines and 4 ACS Paragon Plus Environment

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significantly improve the performances of corresponding PDs. All the procedures were carried out at room temperature in open environments. 2D (OA)2FAn-1PbnBr3n+1 (OA represents octadecylamine, FA is formamidine) microplatelets were drop-casted into pristine film. For treatment, the film was immersed in FA-acetate/acetic acid solution, where cation exchange between OA+ in 2D HPs and FA+ in solution was stimulated. Two desirable improvements were consequently obtained: firstly, the dimensionality of the 2D HPs was increased, which reduces the quantum confinement effect and facilitates the separation of electrons and holes; secondly, the independent 2D microplatelets are fused, forming integrated channels and hence promoting the inter-domain charge carrier transport. As a result, the treated PDs show significantly improved performances compared with the pristine counterparts, the external quantum yield (EQE) and responsivity increased 3600% and 4200% respectively up to 7100% and 32 A/W, and the response time was shortened by more than two orders of magnitude down to 0.25/1.45 ms. We believe this work provides important guidelines for future HPs-based PDs design.



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Figure 1. As-prepared 2D free-standing microplatelets. (a) Schematic illustration of the 2D homologous HPs structure. (b) XRD image and (c) absorption spectrum of as-prepared 2D microplatelets. (d) SEM image showing free-standing 2D microplatelets. (e) TEM image showing single crystal nature of the 2D microplatelets. The inset is the SAED pattern.

Figure 1a depicts the structure of 2D homologous HPs.41,45,47 They can be regarded as intermediates in the transition from 2D to 3D HPs, therefore both structural characteristics of 2D and 3D HPs can be resolved in X-ray diffraction (XRD) image as shown in Figure 1b.41,48,49 A cubic phase of as-prepared 2D microplatelets can be inferred with preferential orientations of (001) and (002).50,51 The long-chain ammonium capping the 2D microplatelets is OA+, which can be confirmed by the 2D characteristic peaks.52 The colloidal dispersion of as-prepared 2D microplatelets shows bright green emission under an UV lamp (Figure S1). However, the absorption spectrum in Figure 1c unravels that as-prepared 2D microplatelets are not homogeneous in dimensionality, few-layered HPs with various numbers of layers 6 ACS Paragon Plus Environment

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exist in the ensemble. The numbers of layers reference that of 2D CsPbBr3 in previous report considering FA+-induced redshift at the same time,53,54 and the absorption at approximately 525 nm represents multilayered 2D microplatelets, of which the number of layers is difficult to identify due to weak quantum confinement effect. The few-layered HPs are undesirable in PDs due to overlarge exciton binding energy which hampers the separation and collection of photogenerated charge carriers.55 The scanning electron microscope (SEM) image in Figure 1d shows that the 2D microplatelets are free-standing with large edge size up to several micrometers. The large edge size is beneficial for long-range carrier transport providing increased mean-free path and reduced hopping, but the stacking of theses independent 2D microplatelets easily causes interfacial charge carrier scattering. The transmission electron microscope (TEM) image in Figure 1e shows well-defined shape of the 2D microplatelets. High crystallinity can be confirmed by the highly ordered selected area electron diffraction (SAED) pattern in the inset.



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Figure 2. Procedures of the treatment and consequent dimensionality increase. (a) Experimental procedures of the treatment. (b) Schematic illustration of the dimensionality increase and (c) consequent change of electronic band structures.

The treatment is schematic illustrated in Figure 2a. A cation exchange reaction was stimulated between OA+ in 2D microplatelets and FA+ in solution. Typically overlarge ammoniums such as OA+ can not fit the cavity within the octahedra framework in 8 ACS Paragon Plus Environment

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HPs and therefore leads to 2D HPs,38,47,52,56 and if these overlarge ammoniums are substituted by suitable cations such as FA+ which correlates to a 3D HPs crystallographic structure,57,58 a dimensionality increase takes place as shown in Figure 2b. Consequently, the electronic band structures change as well. The 2D HPs are self-organized quantum wells as shown in Figure 2c,46,59 the excitons are strongly confined especially in the few-layered HPs and are prone to undergo radiative recombination. However, after the treatment, the self-organized quantum-well electronic band structures are dismissed followed by the reduction of exciton binding energy,46 and charge carriers are easier to separate and be collected.

Figure 3. Optical and structural change brought by the treatment. The evolution of (a) absorption spectra and (b) PL spectra of the films upon various treating time. (c) XRD 9 ACS Paragon Plus Environment


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images of pristine and 30 min-treated films. (d) Comparison of the FWHMs of the XRD images of pristine and 30 min-treated films.

Detailed optical and structural characterizations before and after the treatment are given in Figure 3. The pristine film of 2D microplatelets was treated for various time, the evolution of absorption spectra is presented in Figure 3a. Three features can be clearly observed: firstly, the characteristic absorption peaks of the few-layered 2D HPs gradually weakened and finally disappeared after 30 min treatment; secondly, the absorption onset experienced a red-shift, and finally located at approximately 540 nm; thirdly, the light absorption enhanced in the process especially in the short-wavelength region. These results agree with the dimensionality increase abovementioned, and implies an enhanced light-harvesting capability. It is noteworthy that the long-wavelength absorption is caused by the light scattering rather than real absorption.60 Consistent with the evolving trend of absorption spectra, the PL maxima also undergoes redshift upon prolonged treating time, and a gradual weakening of PL intensity accompanied (Figure 3b), which means a reduction of emissive loss after the treatment. In low-dimensional HPs, the large exciton binding energy increases the possibility of radiative recombination, and dimensionality increase by the treatment causes reduction of exciton binding energy and consequent convenience of separation of electrons and holes. Direct cognition of the treatment is provided in Figure S2 showing pristine and 30 min-treated films under indoor-daylight, and their emissions under an UV-lamp are shown in Figure S3. 10 ACS Paragon Plus Environment

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The change of XRD image after 30 min treatment is shown in Figure 3c. The preferred orientation of pristine 2D microplatelets was inherited by the treated film showing uniaxial (001) and (002) planes. It is noteworthy that recently Kim et al found films with uniaxial orientations own enhanced carrier mobility.29 While such films usually require delicate optimizations of fabrication conditions,29,43 the strategy here is facile, which we believe will provide important methodological guidance for future researches. The diffraction intensity significantly increased approximately 15 times due to the dimensionality transition. The XRD of 2D homologous HPs comprises both in-plane and out-of-plane diffractions, leading to the coexistence of 2D characteristic peaks and 3D peaks, and the treatment here exchanges OA+ with FA+, which causes decrease of out-of-plane diffraction and enhancement of in-plane diffraction. Meanwhile, the 2D characteristic peaks totally disappeared. Moreover, the full-width-at-half-maximun (FWHM) narrowed after the treatment (Figure 3d), which also supports the dimensionality increase caused by the in-situ treatment.



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Figure 4. Inter-microplatelet fusion. (a-b) TEM images of the treated 2D microplatelets. The inset in Figure a shows the single crystal essence of the joint. (c) Lattice match of cubic 2D microplatelets in perpendicular directions. Schematic illustration of the carrier transport in (d) pristine and (e) treated PDs.

Another improvement brought by the treatment is the inter-microplatelet fusion, which constructs integrated channels for charge carriers transport and significantly meliorates the interfacial scattering effect. Figure S4 shows the time-dependent evolution of the microstructures of the film under prolonged treatment. Two features can be observed: firstly, the 2D morphology of the pristine microplatelets was 12 ACS Paragon Plus Environment

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well-preserved. Secondly, the previously independent microplatelets were gradually fused. SAED was then carried out at the joint marked with the red dash circle (Figure 4a), the pattern in the inset shows highly ordered spots, which reveals its single crystalline essence. Therefore, we conclude that FA+ fuses adjacent microplatelets as a whole, forming integrated transporting channels for charge carriers (Figure 4b). An interesting phenomenon is that the 2D microplatelets are perpendicularly fused, this is because the fusion requires lattice match at the interfaces, and for cubic 2D microplatelets, 90o rotation exactly satisfies this requirement (Figure 4c). The inter-microplatelet fusion surely reduces the undesirable interfacial scattering effect and significantly enhances the carrier transport in the PDs as schematically illustrated in Figure 4d and Figure 4e In order to confirm that above effects of dimensionality increase and inter-microplatelet fusion are indeed caused by FA+ rather than by pure acetic acid, we treated the pristine film with pure acetic acid directly, all the other procedures were kept the same and the treating time was set at 30 min. The color of the film barely changed as shown in Figure S5, and the absorption spectrum of the film remained almost unchanged showing multiple-peak feature (Figure S6). Consistently, the SEM image still shows free-standing 2D microplatelets (Figure S7). Therefore, we confirm it is FA+ that plays key role in the treatment.



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Figure 5. Enhanced performances in treated PDs. (a) Photocurrent, (b) I-t curves, (c) EQE and (d) responsivity of the pristine and 30 min-treated PDs.

Both the dimensionality increase and inter-microplatelet fusion contribute to the improvement of charge carrier transport. The conductivities of the treated films with various treating time were determined in the form of planar PDs, the configuration of the PDs is shown in Figure S8. The result shows that the treatment indeed improves the conductivity of the film (Figure S9). Consequently, the performances of the treated PDs are remarkably improved compared with the pristine PDs. In the following the treated PDs represent PDs that were treated for 30 min. Figure 5a shows the I-V curves of pristine and treated PDs under 442 nm illumination of 3.2 mW/mm2. The voltage was swept stepwise from -3 V to 3 V. The treated PDs show more than 14 ACS Paragon Plus Environment

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one order of magnitude higher photocurrent compared with the pristine counterparts, which means an enhanced photoelectric conversion capability. Figure 5b shows the I-t curves of both PDs under 442 nm illumination of 3.2 mW/mm2, the photocurrent of the treated PDs is approximately 20 times as large as that of the pristine PDs. Consistently, the EQE (Figure 5c) and responsivity (Figure 5d) of the treated PDs were also vastly improved, under 9V bias the peak values reach 7100% and 32 A/W, respectively, which are approximately 36 and 42 times as high as those of the pristine PDs. The EQE and responsivity curves are consistent with the absorption curves, and a red-shift of the response onset can be observed in the treated PDs. The magnified EQE and responsivity curves of the pristine PDs against wavelength are shown in Figure S10 and Figure S11.



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Figure 6. Improvement of response speed in treated PDs. (a-b) I-t curves of the pristine PDs under various illumination frequencies. (c-f) The I-t curves of the treated PDs under various illumination frequencies.


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The rise/decay time of the pristine and treated PDs was also measured as a reflection of charge carrier mobility. The rise time is defined as time needed to rise from 10% peak value to 90%, and inverse for decay time. Various illumination frequencies were executed to test their response speed. As shown in Figure 6a and Figure 6b, the response of the pristine PDs was very slow, the I-t curve became triangle-like when 10 Hz illumination was applied, which means the response of PDs couldn’t follow the on/off frequency of illumination. The rise/decay time can be read from Figure 6a to be as long as 78/74 ms. However, for the treated PDs (Figure 6c-f), the current rised sharply when illuminated, and rapidly dropped when sheltered. The I-t curve is still well-defined under a high frequency of 300 Hz, from which the rise/decay time is determined to be 0.25/1.45 ms. In conclusion, we have reported an innovative post-synthetic treatment to simultaneously engineer the dimensionality and interface of 2D homologous HPs and significantly

improve

(OA)2FAn-1PbnBr3n+1

the

performances

of

corresponding

PDs.

2D

microplatelets film was immersed in FA+ solution, where

exchange of OA+ with FA+ was stimulated and led to two improvements: firstly, a dimensionality increase was triggered to meliorate the quantum confinement effect and facilitates the separation of electrons and holes; secondly, the independent 2D microplatelets were fused, removing undesirable interfacial scattering. Therefore, the treated PDs showed significantly improved performances compared with the pristine counterparts, the EQE and responsivity increased 3600% and 4200% up to 7100% and 32 A/W, respectively, and the rise/decay time was shortened by two orders of 17 ACS Paragon Plus Environment


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magnitude down to 0.25/1.45 ms. We believe this work provides important guidelines for future HPs-based device construction from the viewpoint of intrinsic material properties.

AUTHOR INFORMATION E-mail: [email protected] (H. Zeng)

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENT D. Yu, F. Cao and Y. Shen contributed equally to this work. This work was financially supported by the National Key Basic Research Program of China (2014CB931702), NSFC (51572128, 21403109, 51502139), NSFC-RGC (5151101197), the National Key Research and Development Program of China (2016YFB0401701), the Fundamental Research Funds for the Central Universities (No. 30915012205, 30916015106), Natural Science Foundation for Youths of Jiangsu Province of China (BK20140787) and PAPD of Jiangsu Higher Education Institutions, and in USA by DoD (Grant W911NF-15-1-0650).

Supporting Information As-prepared 2D microplatelets dispersion, pristine and treated films under indoor 18 ACS Paragon Plus Environment

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light, pristine and treated films under UV excitation, time-dependent SEM images of the treated films, pristine and acetic acid-treated films, absorption spectrum of acetic acid-treated film, SEM image of acetic acid-treated film, schematic illustration of the PDs, time-dependent conductivity of the film, EQE of pristine PDs, responsivity of the pristine PDs.

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Dimensionality and Interface Engineering of 2D Homologous

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